[R] Applying function to parts of a matrix based on a factor

[Heinz Tuechler]

If your matrix were a data.frame, it could work like this:

df <- data.frame(age=1:100, sex=rep(1:2, 50))
with(df, by(age, sex, mean))

without the "lapply, sapply etc. family".

h

At 18:16 13.01.2010, Doran, Harold wrote:
>with(yourdataframe, tapply(age,sex,mean))
>
>-----Original Message-----
>From: r-help-bounces at r-project.org 
>[mailto:r-help-bounces at r-project.org] On Behalf Of John Sorkin
>Sent: Wednesday, January 13, 2010 12:11 PM
>To: r-help at r-project.org
>Subject: [R] Applying function to parts of a matrix based on a factor
>
>R 2.9
>Windows XP
>
>I have a matrix, Data, which contains a factor Sex and a continuous 
>variable Age.
>I want to get mean age by sex. I know I can do this with two statements,
>mean(Data["Age,Data[,"Sex"]=="Male") and
>mean(Data["Age,Data[,"Sex"]=="Female")
>
>I know this can be done in a single command, but I can remember how. 
>There is a function that allows another function work within 
>factors, something like
>magicfunction(Data,Factor=Sex). n.b. I know the function I am 
>looking for is not in the lapply, sapply etc. family
>
>Please put me out of my misery (and senior moment) and remind me 
>what function I should be using.
>
>
>
>
>John David Sorkin M.D., Ph.D.
>Chief, Biostatistics and Informatics
>University of Maryland School of Medicine Division of Gerontology
>Baltimore VA Medical Center
>10 North Greene Street
>GRECC (BT/18/GR)
>Baltimore, MD 21201-1524
>(Phone) 410-605-7119
>(Fax) 410-605-7913 (Please call phone number above prior to faxing)
>
>Confidentiality Statement:
>This email message, including any attachments, is for t...{{dropped:9}}